Not Applicable.
Not Applicable.
The present embodiments relate to wireless communication systems, and are more particularly directed to such systems using frequency hopping.
Wireless networks are becoming increasingly popular, and in this regard there has been improvement in many aspects of such networks. Some improvements relate to configurations that permit simultaneously operation of different networks where there is minimal or no interference between communications belonging to each of the networks. In this respect, the term network is used, and is further used in the same manner for the remainder of this document, to describe a system consisting of an organized group of intercommunicating devices. Further in this respect, the different networks may be labeled according to a first network that is already transmitting in time followed by a second network in time seeking to transmit and thereby possibly communicating and causing interference due to a communication overlapping the pre-existing communication of the first network. Accordingly, to facilitate the remaining discussion, such a first network is referred to as an incumbent network, while the network which seeks to communicate, or in fact does communicate, after the incumbent network is referred to as the newly-entering network. Given this terminology, the present background and embodiments discussed below are directed to reducing interference between incumbent network communications and newly-entering network communications.
One approach to reducing the above-introduced interference is known in the art as spread spectrum frequency hopping and is sometimes referred to more simply as frequency hopping. In frequency hopping, a newly-entering network transmitter transmits packets of information at different frequencies in an effort to reduce the chance that the packet will interfere or xe2x80x9ccollidexe2x80x9d with a packet transmitted at a frequency by a transmitter in an incumbent network. The change between frequencies, that is, from one frequency to another, is said to be a xe2x80x9chopxe2x80x9d between the frequencies. Moreover, the goal is such that each packet from a newly-entering network is transmitted at a frequency which neither overlaps nor is near enough to a frequency at which an incumbent network is transmitting. Further in this regard, some systems (e.g., using Bluetooth protocol) transmit each successive packet at a different frequency, that is, the transmitter is xe2x80x9choppingxe2x80x9d to a different frequency for each packet. Alternatively, others systems (e.g., IEEE 802.11) transmit a first set of packets at a first frequency, and then hop to a second frequency to transmit a second set of packets, and so forth for numerous different sets of packets at numerous different respective frequencies. Note further that if interference or a collision does occur, it typically corrupts the data of both packets, that is, the data transmitted by both the newly-entering network and the incumbent network. As a result, both networks are then required to re-transmit the packets an additional time so as to replace the corrupted data resulting from the collision.
In an effort to achieve minimal packet collision using frequency hopping, two prior art methods have arisen for determining the different frequencies to which a network will hop. In a first method, a frequency hopping network uses a pre-ordained hopping sequence. This first approach is used by way of example under the IEEE 802.11 standard. In a second method, a seed is provided to a pseudo-random generator which produces a corresponding pseudo-random series of frequencies along which the network hops. This second approach is used by way of example under the fairly recently developed Bluetooth protocol. Both of these approaches have achieved some level of success in reducing the amount of inter-network packet collision. Nevertheless, the present inventors have empirically determined that by locating two or more different networks in the same vicinity such that transmissions from each different network effectively compete for airtime, there still arises a considerable amount of packet collisions, thereby reducing the effective transmission rate for each network.
Frequency hopping as described thus far reduces the chances of interference between a packet from newly-entering network and a packet from an incumbent network. Further in this regard and by way of additional background, FIG. 1 illustrates communications of such packets and, as detailed below, it also illustrates instances where packet collisions occur. Looking to FIG. 1 in greater detail, its horizontal axis illustrates time (or time slots), and its vertical axis indicates frequency. Additionally, FIG. 1 illustrates a number of blocks, where each block is intended to depict a packet as transmitted by either an incumbent network or a newly-entering network. Further in this regard, note that the term xe2x80x9cpacketxe2x80x9d is used in this document to define a block of information sent in a finite period of time, where subsequent such packets are sent at other times. This block of information may take on various forms, and sometimes includes different information types such as a preamble or other type of control information, followed by user information which is sometimes also referred to as user data. Further, the overall packet also may be referred to in the art by other names, such as a frame, and thus these other information blocks are also intended as included within the term xe2x80x9cpacketxe2x80x9d for purposes of defining the present inventive scope. In any event, returning to FIG. 1, for the sake of reference, each packet illustrated in FIG. 1 is labeled with an identifier using the letter xe2x80x9cPxe2x80x9d (i.e., for packet) and following after that letter is a number corresponding to the network which transmitted the packet. More particularly, packets transmitted by the first network (i.e., the incumbent network) are labeled with an identifier P1 while packets transmitted by the second network (i.e., the newly-entering network) are labeled with an identifier P2. Further, the subscript for each packet identifies a time period encompassed by the duration of the packet. For example, during a time to, the first network transmits a packet P10 while also during time t0 the second network transmits a packet P20. Further in this regard, in the prior art transmissions by the first network are asynchronous with respect to transmissions of the second network, both in start time and periodicity. Thus, time t0 is only meant as a relative indication for the first packet from each network, and it is not intended to suggest that the packets from both networks begin and end at the same time.
With respect to all packets in FIG. 1, the preceding demonstrates that each packet begins at a certain time, ends at a later time, and fills a certain frequency range (where the range is referred to as a channel). As a result and as described below, interference may occur if the area in FIG. 1 defined by a packet overlaps or is within a certain distance of a packet from another wireless link. Indeed and as discussed below, such interference may occur in one of four different ways.
Time t1 in FIG. 1 illustrates a first type of packet interference, where it may be seen that the first network transmits a packet P11 After packet P11 commences but also during time t1 the second network transmits a packet P21. The overlap of packets P11 and P21 is shown as a first collision C1. Note that the horizontal alignment of packets P11 and P21 graphically indicates that in the example of collision C1, both packets occupy the same frequency channel. Thus, collision C1 represents an example where two different networks attempt to transmit packets during an overlapping time period and along the same channel.
Before proceeding with other types of packet collisions, an additional discussion is noteworthy with respect to a methodology which has been used to further reduce the likelihood and impact of packet collisions such as collision C1. More particularly, this additional methodology is referred to in the art as listen-before-talk (xe2x80x9cLBTxe2x80x9d). In an LBT system, the system uses the hopping sequence described above, but prior to transmitting along a channel in the sequence the system monitors (or xe2x80x9clistensxe2x80x9d) at the channel to determine if there is another packet already occupying that channel during the current time. Returning to packet P11 by way of example, if the second network employed LBT, then it would listen at the desired channel at which it intended to transmit P21 and would therefore detect the presence of packet P11. As a result, the second network would avoid collision C1 by not transmitting packet P21 at the desired frequency, but instead it would delay a random period and then proceed to the next designated channel of its hopping sequence. Next, the second network would listen at that next designated channel to again determine if that channel was occupied by a packet from another network, and if no packet was detected then the second network would transmit its packet; however, if this next designated channel also was occupied, then the second network would continue to examine additional channels in this same manner until a channel was detected without being occupied by a packet from another network, at which time the second network would transmit its packet along the now unoccupied channel. Given this process, however, note that a delay arises in LBT systems, where the amount of delay depends on the number of times that the LBT network is forced to listen, detect, and advance from an occupied channel, and then delay an additional random period to listen, detect, and transmit along an unoccupied channel.
While LBT as shown above reduces the possibility of collisions, it also has drawbacks. For example, LBT delays transmission by the network which was prepared to transmit along a channel but was prevented from doing so due to an already-transmitted packet in the desired channel. As another example, it adds an element of delay to each packet due to its listening aspect. Also, all the devices in an environment must utilize LBT to gain the most benefit (fairness) of the scheme. As still another example, some protocols (e.g., Bluetooth) utilized in the unlicensed bands do not support LBT, while such protocols may nonetheless provide other beneficial aspects and, thus, the choice to use such a protocol is a tradeoff in that other aspects are obtained without the availability of LBT.
Time t2 in FIG. 1 illustrates a second type of packet interference in connection with a collision C2 occurring between a first network packet P12 and a second network packet P22. For collision C2, the incumbent first network transmits packet P12 during a period including time t2 and at a first channel, and thereafter the second network transmits packet P22 also during a period including time t2 (i.e., the periods of the packets overlap). Packet P22 is transmitted at a second channel which, while different than the channel of packet P12, it is immediately adjacent the channel occupied by packet P12. Further in this regard, it is known in the art that while packets occupy a certain channel as shown by the vertical displacement of a packet in FIG. 1, there is an additional tendency for a packet to provide slight interference or xe2x80x9csplatterxe2x80x9d into adjacent frequency channels. As a result of this effect, even though packets P12 and P22 occupy different channels, they are still in adjacent channels and, thus, they are close enough to one another in frequency such that the splatter effect causes a collision between the packets. Indeed, in some networks the filters used are relatively inexpensive and, as a result, the concept illustrated with packets P12 and P22 may also apply to next-adjacent channels, that is, to the channels that are one more channel away from the channels adjacent to the channel in which a packet is transmitted. Thus, collision C2 represents an example where two different networks attempt to transmit packets during an overlapping time period and along adjacent (or next-adjacent) frequency channels. Here, if neither network uses LBT, then both packets P12 and P22 will require retransmission due to the collision. If, however, the network that intended to transmit the second packet of the two uses LBT, then note first that LBT mechanisms are less likely to correctly discern an adjacent channel collision. However, if the LBT mechanism does recognize the potential adjacent channel collision, then the second packet is not transmitted along the channel represented by P22 and instead that packet is delayed. This delay, while diminishing the effective transmission of the second network, avoids any disturbance to the first already-existing packet. In the example of time t2, therefore, if the second network uses LBT, then packet P12 will not be disturbed because the second network will move the transmission of packet P22 to a different channel.
Time t4 in FIG. 1 illustrates a third type of packet interference in connection with a collision C4, which is comparable to collision C2 except that for collision C4 the networks transmit in opposite order. More particularly, for collision C4, the second network first transmits a packet P24 and, thereafter, the first network transmits a packet P14. The duration of both of these packets overlaps time t4, and again their channels are adjacent to one another rather than being the same channel. Nonetheless, the splatter effect again causes sufficient reach of each packet into the adjacent channel such that a collision occurs. Here, if neither network uses LBT, then both packets P24 and P14 require re-transmission due to the collision; if, however, the network transmitting the second packet in time (i.e., P14) of the two which would otherwise collide uses LBT, then only that packet is delayed and the first already-existing packet (i.e., P24) is not disturbed.
Time t7 in FIG. 1 illustrates a fourth type of packet interference in connection with a collision C7, which is comparable to collision C1 except that for collision C7 the networks transmit in opposite order. More particularly, for collision C7, the second network first transmits a packet P27 and, thereafter, the first network transmits a packet P17. The duration of both of these packets overlap time t7 and their channels are the same. As a result, collision C7 occurs (assuming the last network to transmit, which here is the first network, does not use LBT).
FIG. 1 illustrates an additional type of potential interference by depicting a band of fixed interference FI. Fixed interference FI is intended to represent a non-network source of radio frequency transmission that remains at the same frequency for numerous time slots. Such fixed interference may arise from various devices, such as a leaking microwave oven by way of example. In any event, note at time t5 that the second network transmits a packet P25, and the channel along which that packet is transmitted overlaps fixed interference FI. As a result, fixed interference FI interferes with packet P25, thereby requiring it to be re-transmitted. Once more, however, if the second network were to implement LBT, then assuming fixed interference FI were detected during the listening operation of the LBT, then packet P25 would not be transmitted so as to avoid the otherwise imminent interference. Lastly, while the example of packet P25 demonstrates a data collision where the packet uses the same channel as the fixed interference, note further that fixed interference also may disturb packets in a channel that is adjacent to the channel including the fixed interference. Once more, because some networks use relatively inexpensive filters, the fixed interference may corrupt packets which are either in a channel which is immediately adjacent to the fixed interference or which are in the next-adjacent channel (i.e., a channel which is next to the channel that is immediately adjacent to the fixed interference).
In view of the above, one skilled in the art should appreciate there are various opportunities for packet collision or packet interference to occur. Indeed, referring to FIG. 1, the examples above demonstrate that an area may be described around each packet, where the packet is disturbed if another packet occurs within that area. Thus, this area, which may be perceived as a window or zone around the packet, is not only defined by the dimensions of the packet, but extends both before and after the packet by the width of another potentially-interfering packet, and extends above and below the packet channel through the height of at least the adjacent channel above and below the packet frequency channel. Still further, note that the packet sizes for both networks shown in FIG. 1 are the same size by way of example; however, in some contexts, an incumbent network may use packets of different dimension (i.e., either in frequency and/or time) relative to the newly-entering network. In these cases, the packet size for the incumbent as well as the packet size for the newly-entering network, in addition to the window-affecting factors described above, all further define a two-dimensional area relative to a newly-entering packet in which interference may occur. Given the size of the two-dimensional area, therefore, there remains a possibility of packet disturbance even given the pseudo random nature of hopping spread spectrum RF communications.
As an additional consideration relative to avoiding packet collisions, it is further noted that the Federal Communications Commission (xe2x80x9cFCCxe2x80x9d) imposes a restriction on the art in the Industrial Scientific Medical (xe2x80x9cISMxe2x80x9d) bands. Specifically, the FCC explicitly forbids independent networks to expressly cooperate in allocation of the wireless medium.
In view of the above, there arises a need to reduce the possibility of packet collision and interference, and preferably to do so in a manner that may be used with protocols that do not support LBT. The preferred embodiment addresses these goals, as is explored below. In addition, there arises a need to achieve the above goals while complying the with the above-described FCC requirements. The preferred embodiments described below avoid these requirements by not requiring the newly entering network to have knowledge of or cooperation with the incumbent network.
In the preferred embodiment, there is a method for determining a frequency hopping sequence for a newly-entering network. The method comprises the step of scanning a plurality of frequency channels. For each of the plurality of frequency channels, the scanning step comprises detecting whether a signal exists on the channel and recording information corresponding to each channel on which a signal is detected. Finally, and responsive to the recorded information, the method forms the frequency hopping sequence. Other circuits, systems, and methods are also disclosed and claimed.